The invention relates to a sample changer for a balance, in particular for a comparator balance. The sample changer serves to transport the weighing objects to a weighing pan and has at least two sample carriers to hold the objects to be weighed. The sample carriers are arranged at a distance from each other on the sample changer.
A sample changer for a comparator balance is disclosed in DE-U-295 17 368. The sample changer described there serves to consecutively transport the samples, which in this particular case are test weights, to a weighing pan and to subsequently remove the samples from the weighing pan. The weighing object is placed on a receiver stage that is perforated by slots and can be lowered and lifted in relation to the weighing pan. The weighing pan has an arrangement of vertically upstanding ridges that pass through the slots of the receiver stage when the latter is lowered and lifted. The weighing pan with the arrangement of ridges consists of at least four branches that are joined by a connecting ridge. With a comparator balance of this design, it is possible to also weigh several weights of different sizes together with the highest degree of accuracy.
Comparator balances are used to make accurate measurements of the mass and volume of weight pieces. Their application is primarily in governmental institutes of metrology. When making comparison weighings, it is often necessary to handle test weights in a diversity of different sizes. A principal requirement for these kinds of balances is that they have to be capable of making very accurate weight measurements. It is therefore important that the weights under test not be exposed to changes in the ambient conditions, in particular to air drafts, during a weighing process.
A sample-handling device for analytical applications, which by means of a gripping device transports the sample under analysis to a balance and subsequently transfers the sample from the balance to an analytical instrument, is described in DE-A-38 27 293. To avoid the problem of air drafts, the weighing takes place in an enclosed space that has to be opened to place the sample on the weighing pan and to subsequently take the sample out again.
The last-mentioned among the devices known under the existing state of the art has the disadvantage, that the samples are exposed to the ambient atmosphere during the times when they are being transported to or from the balance pan and that they could therefore have a temperature that is different from the enclosed space above the weighing pan. This risk exists in particular in a case where the sample has to be transported over relatively large distances to reach the weighing pan. For weighings of the highest precision, it could become necessary to allow an acclimating period before the actual weighing, but this is a time-consuming measure.
It is therefore the object of the present invention to propose a sample changer which, in cooperation with a balance, provides the most stable environment possible for the weighing objects, in particular with regard to avoiding air currents and keeping the temperature constant, and which thereby creates conditions conducive to accurate weighing.
According to the present invention, a sample changer for a balance with a weighing pan has at least two sample carriers arranged at a distance from each other and designed so that each of the sample carriers can support at least one unit of the objects to be weighed and transport the weighing-object unit to the weighing pan. An individual draft-shield element, made at least in part of a transparent material, is provided for each of the sample carriers, enclosing the top and the sides of a sample space.
With the sample changer according to the invention, it is not necessary to open the draft-shield when transporting a new sample to the weighing pan, as the sample changer brings the weighing objects directly to the weighing pan without taking them out of the sample space that is surrounded by the draft shield, so that the weighing objects don""t have to leave the environmental conditions that are favorable for weighing. Rather, each of the sample carriers together with its own draft-shield element is movable in relation to the weighing pan to bring the at least one weighing object into a weighing position. This arrangement also assures a stable temperature for the weighing-object units that are enclosed in the sample space.
In a preferred embodiment, the sample changer has a platform to which at least two sample carriers can be attached by means of a releasable connection. To transport the at least one weighing object to the weighing pan, the platform is movable in relation to the weighing pan.
When a sample carrier is in the weighing position, the weighing pan will be located inside the sample space enclosed by the draft-shield element that belongs to that sample carrier. To transfer the weighing-object units to the weighing pan, the platform and/or the sample carrier are movable up and down in relation to the weighing pan.
In an advantageous embodiment of the invention, a draft-shield element is configured essentially as a hollow body consisting in particular of glass. The draft-shield element has a seating rim. To define a stable position of the draft-shield element on the sample changer, the seating rim of the draft-shield element sits in a matching seating recess of the sample changer. Both the seating rim and the seating recess can be of a polygonal shape, with the seating rim fitting the shape of the seating recess of the sample changer.
In a particularly advantageous development of the invention, each of the draft-shield elements has a gripping device by which the draft-shield device can be handled, in particular for putting the draft-shield device on and off the sample changer.
Holders for sensors are installed in recesses of the platform of the sample changer and extend into the sample spaces. The purpose of the sensors is to measure environmental parameters, e.g., temperature or humidity, for the weighing-object units inside the sample space.
In a particularly preferred embodiment according to the invention, the sample carrier is configured as a grating. In the weighing position of the sample carrier, a weighing pan with upstanding ridges reaches through the grating. In an advantageous version of the invention, the sample changer may have a circular platform that is rotatable about its center point and has four receiving positions arranged at 90xc2x0 from each other for the sample carriers. This arrangement is particularly advantageous for a comparator balance.